This invention relates to a method of determining the acceptability of an exhaust concentration sensor arranged in the exhaust system of an internal combustion engine for sensing the concentration of an emission component contained in the engine exhaust.
A commonly employed air-fuel ratio feedback control system for internal combustion engines is described, for example, in Japanese Provisional Patent Publication (Kokai) No. 57-137633. In accordance with this conventional system, the air-fuel ratio of the mixture supplied to an internal combustion engine is subjected to feedback control to obtain a stoichiometric mixture ratio that will maximize the conversion efficiency of a three-way catalyst arranged in the engine exhaust system in such a manner that the amount of fuel supplied to the engine is set to a basic value dependent, for example, upon the rotational speed of the engine and the absolute pressure in the engine intake pipe, the basic value being corrected by a correction value which conforms to parameters inclusive of the concentration of oxygen contained in the engine exhaust sensed by an exhaust concentration sensor (hereinafter referred to as an "O.sub.2 sensor") arranged in the engine exhaust system.
The O.sub.2 sensor employed in the above feedback control system uses a substance such as zirconium oxide as a sensing element. Utilizing the fact that the amount of oxygen ion which permeates the interior of the zirconium oxide varies depending upon the difference between the partial pressure of oxygen in the atmosphere and the partial pressure of oxygen contained in the exhaust gas, the O.sub.2 sensor senses oxygen concentration in the exhaust gas to output a voltage which varies as a function of the above-mentioned variation in partial pressure difference.
If the O.sub.2 sensor employed in the above-described air-fuel ratio feedback control system should happen to be defective, the air-fuel ratio of the mixture supplied to the engine will indicate an abnormal value and, as a result, appropriate control of the engine will not be possible.
Accordingly, methods of determining whether the O.sub.2 sensor is faulty have been proposed. Basically, these conventional methods are of two types. One method proposed by Japanese Provisional Patent Publication (Kokai) No. 54-5129 judges the O.sub.2 sensor to be faulty when the value of its output voltage departs from a range defined by the maximum and minimum values that the sensor can exhibit when operating normally. The other method proposed by U.S. Pat. No. 3,916,848 relies upon the so-called "changeover" of the O.sub.2 sensor, in which the sensor output voltage level changes from the rich side to the lean side, or vice versa, with respect to a predetermined reference value. According to this method, the O.sub.2 sensor is judged to be defective when the changeover fails to occur within a predetermined period of time.
With the former method, the O.sub.2 sensor will not be judged to be abnormal even if it possesses an output characteristic in which the sensor output voltage is offset to the rich or lean side, so long as the output voltage level of the sensor lies within the range defined by the maximum and minimum values. Using an O.sub.2 sensor having such a characteristic will cause the air-fuel ratio of the mixture to shift toward the lean or rich side. Similarly, the latter method will not find the O.sub.2 sensor to be faulty even if it has the above-mentioned output characteristic favoring the rich or lean side, so long as the changeover of the O.sub.2 sensor output occurs within the predetermined time period.
Conventionally, it is general practice to install an O.sub.2 sensor, which has been found acceptable through one of the aforementioned methods, in the engine exhaust system on an automotive vehicle mass-production line and then actually run the completed vehicle to measure the amount of toxic components in the exhaust gas by the so-called "10 mode test method". Often an engine using an O.sub.2 sensor judged to be acceptable in the manner set forth above will, based on the results of the test, fail to satisfy the prescribed limit values on HC or NOx emissions contained in the engine exhaust gas. When such is the case, the O.sub.2 sensor must be dismounted from the vehicle and replaced. This requires considerable time and labor.